Resistive-sensor interface

Abstract

A device to read a variable resistor has an analog to digital converter (ADC), a first switch and a second switch. The ADC has a first ADC input, a second ADC input and an ADC output. The first switch selectively couples a first voltage indicative of a voltage across a first resistance to the first ADC input. The second switch selectively couples a second voltage indicative of a voltage across a second resistance to the second ADC input. The ADC outputs a signal indicative of a value of the second resistance.

The invention claimed is: 
     
       1. A device, comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a first voltage indicative of a voltage across a first resistance and selectively couples the first switch input to the first ADC input; and 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across a second resistance and selectively couples the second switch input to the second ADC input, wherein the ADC, in operation, outputs at the ADC output a signal indicative of a value of the second resistance, wherein: 
 said ADC comprises two negative feedback loops, which, in operation, function according to a value of the first switch output and a value of the second switch output, 
 said two negative feedback loops comprise first-order continuous-time sigma-delta converters, 
 the ADC comprises an integrator, and each sigma-delta converter comprises a comparator and a sequential electronic circuit coupled to the integrator, and 
 said first switch and said second switch are multiplexers having respective selection inputs coupled to respective outputs of said sequential electronic circuits of the sigma-delta converters, and, in operation, the outputs of the sequential electronic circuits provide negative feedback of the negative feedback loops. 
 
     
     
       2. The device of  claim 1  wherein the signal indicative of the value of the second resistance is a ratio of a value of the first voltage and a value of the second voltage. 
     
     
       3. The device of  claim 1  wherein said sequential electronic circuits are D flip-flops. 
     
     
       4. A device, comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output, wherein the ADC includes an integrator and two negative feedback loops comprising first-order continuous-time sigma-delta converters, each sigma-delta converter comprising a comparator and a sequential electronic circuit coupled to the integrator; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a first voltage indicative of a voltage across a first resistance and selectively couples the first switch input to the first ADC input; and 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across a second resistance and selectively couples the second switch input to the second ADC input, wherein the ADC, in operation, outputs at the ADC output a signal indicative of a value of the second resistance, and the two negative feedback loops, in operation, function according to a value of the first switch output and a value of the second switch output, and wherein the device comprises: 
 low-pass filters coupled to respective outputs of said sequential electronic circuits; and 
 a digital divider having respective inputs coupled to respective outputs of the low-pass filters, wherein the digital divider, in operation, outputs a digital representation of the value of the second resistance as a ratio between values representing said first voltage and said second voltage. 
 
     
     
       5. The device according to  claim 4  wherein said first switch and said second switch are multiplexers having respective selection inputs coupled to respective outputs of said sequential electronic circuits of the sigma-delta converters, and, in operation, the outputs of the sequential electronic circuits provide negative feedback of the negative feedback loops. 
     
     
       6. A system, comprising:
 a sensor including a variable resistor; and 
 circuitry coupled to the sensor, which, in operation, generates a signal indicative of a resistance of the variable resistor, the circuitry comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a reference voltage and selectively couples the first switch input to the first ADC input; and 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across the variable resistor and selectively couples the second switch input to the second ADC input, wherein 
 
 said ADC comprises:
 an integrator; and 
 two negative feedback loops, said two negative feedback loops being first-order continuous-time sigma-delta converters having a comparator and a sequential electronic circuit coupled to the integrator; and 
 
 said first switch and said second switch are multiplexers having selection inputs coupled to respective outputs of said sequential electronic circuits of the sigma-delta converters, and, in operation, the outputs of the sequential electronic circuits provide negative feedback of the negative feedback loops. 
 
     
     
       7. The system of  claim 6  wherein the circuitry comprises:
 low-pass filters coupled to respective outputs of said sequential electronic circuits; and 
 a digital divider having inputs coupled to respective outputs of the low-pass filters, wherein the digital divider, in operation, outputs a digital signal indicative of the resistance of the variable resistor. 
 
     
     
       8. A system, comprising:
 a sensor including a variable resistor; and 
 circuitry coupled to the sensor, which, in operation, generates a signal indicative of a resistance of the variable resistor, the circuitry comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a reference voltage and selectively couples the first switch input to the first ADC input; and 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across the variable resistor and selectively couples the second switch input to the second ADC input, wherein 
 
 said ADC comprises:
 an integrator; and 
 two negative feedback loops, said two negative feedback loops being first-order continuous-time sigma-delta converters having a comparator and a sequential electronic circuit coupled to the integrator; and 
 
 the circuitry comprises:
 low-pass filters coupled to respective outputs of said sequential electronic circuits; and 
 a digital divider having inputs coupled to respective outputs of the low-pass filters, wherein the digital divider, in operation, outputs a digital signal indicative of the resistance of the variable resistor. 
 
 
     
     
       9. The system of  claim 8  wherein said sequential electronic circuits are D flip-flops. 
     
     
       10. A device, comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output, wherein the first switch, in operation, receives at the first switch input a first voltage indicative of a voltage across a first resistance and selectively couples the first switch input to the first ADC input; 
 a second switch having a second switch input and a second switch output, wherein the second switch, in operation, receives at the second switch input a second voltage indicative of a voltage across a second resistance and selectively couples the second switch input to the second ADC input, wherein the ADC, in operation, outputs at the ADC output a signal indicative of a value of the second resistance; 
 first and second multiplexers, each having:
 an input coupled to the ADC output; 
 an input, which, in operation, has a fixed value; 
 a selection input; and 
 
 a comparator, which in operation, compares the first voltage and the second voltage and generates a control signal based on the comparison, the comparator having an output coupled to the selection inputs of the first and second multiplexers. 
 
     
     
       11. The device of  claim 10 , comprising:
 a digital-divider having inputs coupled to respective outputs of the first and second multiplexers, wherein in operation, the digital divider generates a digital representation of a value of the second resistance. 
 
     
     
       12. The device of  claim 10  wherein, in operation, the first and second switches are controlled based on the control signal generated by the comparator. 
     
     
       13. The device of  claim 12  wherein,
 the first switch has a third switch input, and the first switch, in operation, receives at the third switch input the second voltage and selectively couples the third switch input to the first ADC input; and 
 the second switch has a fourth switch input, and the second switch, in operation, receives at the fourth switch input the first voltage and selectively couples the fourth switch input to the second ADC input. 
 
     
     
       14. A system, comprising:
 a sensor including a variable resistor; and 
 circuitry coupled to the sensor, which, in operation, generates a signal indicative of a resistance of the variable resistor, the circuitry comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a reference voltage and selectively couples the first switch input to the first ADC input; 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across the variable resistor and selectively couples the second switch input to the second ADC input; 
 a comparator, which in operation, compares the reference voltage and the second voltage and generates a control signal based on the comparison; and 
 two output multiplexers, each having an input coupled to the ADC output, an input, which, in operation, has a fixed value, and a selection input coupled to the output of the comparator. 
 
 
     
     
       15. The system of  claim 14  wherein the circuitry comprises:
 a digital-divider having inputs coupled to respective outputs of the output multiplexers, wherein in operation, the digital divider generates a digital representation of a resistance of the variable resistor. 
 
     
     
       16. The system of  claim 15  wherein, in operation, the first and second switches are controlled based on the control signal generated by the comparator. 
     
     
       17. A device, comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output, wherein the first switch, in operation, receives at the first switch input a first voltage indicative of a voltage across a first resistance and selectively couples the first switch input to the first ADC input; 
 a second switch having a second switch input and a second switch output, wherein the second switch, in operation, receives at the second switch input a second voltage indicative of a voltage across a second resistance and selectively couples the second switch input to the second ADC input, wherein the ADC, in operation, outputs at the ADC output a signal indicative of a value of the second resistance; 
 a comparator, which in operation, compares the first voltage and the second voltage and generates a control signal based on the comparison; 
 a digital divider having an input coupled to the output of the ADC; and 
 an output multiplexer having a first input coupled to the output of the ADC, a second input coupled to an output of the digital divider, and a selection input coupled to an output of the comparator. 
 
     
     
       18. The device of  claim 17  wherein, in operation, the first and second switches are controlled based on the control signal generated by the comparator. 
     
     
       19. The device of  claim 17  wherein,
 the first switch has a third switch input, and the first switch, in operation, receives at the third switch input the second voltage and selectively couples the third switch input to the first ADC input; and 
 the second switch has a fourth switch input, and the second switch, in operation, receives at the fourth switch input the first voltage and selectively couples the fourth switch input to the second ADC input. 
 
     
     
       20. A system, comprising:
 a sensor including a variable resistor; and 
 circuitry coupled to the sensor, which, in operation, generates a signal indicative of a resistance of the variable resistor, the circuitry comprising:
 an analog to digital converter (ADC) having first ADC input, a second ADC input and an ADC output; 
 a first switch having a first switch input and a first switch output and which, in operation, receives at the first switch input a reference voltage and selectively couples the first switch input to the first ADC input; 
 a second switch having a second switch input and a second switch output and which, in operation, receives at the second switch input a second voltage indicative of a voltage across the variable resistor and selectively couples the second switch input to the second ADC input; 
 a comparator, which in operation, compares the reference voltage and the second voltage and generates a control signal based on the comparison; 
 a digital divider having an input coupled to the output of the ADC; and 
 an output multiplexer having a first input coupled to the output of the ADC, a second input coupled to an output of the digital divider, and a selection input coupled to an output of the comparator. 
 
 
     
     
       21. The system of  claim 20  wherein, in operation, the first and second switches are controlled based on the control signal generated by the comparator. 
     
     
       22. The system of  claim 20  wherein the sensor includes a reference resistor and the reference voltage is a voltage across the reference resistor.